Diisocyanates which contain urea groups are generally distinguished by a relatively high melting point. Due to the low vapor pressure thereof and due to the presence of the urea groups, they are preferably used as starting materials for the production of high-grade polyurethane/polyurea plastics. These diisocyanates are usually produced by reacting 2 mols of diisocyanate with 1 mol of water in an inert solvent. It is preferred to use diisocyanates which have two differently-reactive groups, when a definite reaction course is to be achieved. Thus, French Pat. No. 1,103,329 describes the production of bis-(3-isocyanato-4-alkoxy-, -alkyl- or chloro-phenyl) ureas by a reaction of diisocyanate and water in a molar ratio of 2:1, in a solvent, which solvent must not contain reactive hydrogen atoms (for example in the form of hydroxyl groups). Ethyl acetate, diethyl ether or acetone are preferably used as solvent.
U.S. Pat. No. 3,906,019 describes a process for the production of monomeric di-(isocyanatotolyl)-urea, in which the reaction is carried out in an excess of one of the two non-miscible reactants (toluylene diisocyanate and, preferably, water) at from 20.degree. to 40.degree. C. The urea which is not soluble in either of the starting materials is separated in a yield of from about 60 to 85%, mainly in monomeric form, and it must then be purified. Lewis acids or Lewis bases may be used as catalysts for the reaction described therein, but pyridine is preferred.
All these processes are carried out in solvents. The diisocyanates must be very soluble and the water which is added must be at least partly miscible in the solvents. The solvent must not exert a polymerizing effect on the isocyanate and must be free from isocyanate-reactive functional groups. The urea diisocyanates are produced as relatively insoluble compounds and are isolated by filtration. For further processing of these urea isocyanates for polyurethane production, it is necessary to convert the urea isocyanates obtained by filtration (and freed from solvents under vacuum) into a finely-divided form by a suitable grinding operation. As a result of the high melting point and the difficult solubility of these isocyanate ureas, inhomogeneous products are often obtained during the reaction.
According to German Auslegeschrift No. 2,902,469, suspensions of (poly)-urea-diisocyanates in isocyanate-prepolymers are produced by dissolving 1 mol of aromatic diisocyanate in the NCO-prepolymer and reacting it with from about 0.4 to 0.8 mols of water. Organic diisocyanates which have differently-reactive NCO groups are used to produce the NCO-prepolymers, as well as to produce the (poly)urea diisocyanates.
No process has been known in which a polyisocyanate containing urea groups is directly produced by reacting organic polyisocyanates with water in an NCO-reactive organic solvent, for example long-chain hydroxyl polyethers or hydroxy polyesters. According to the prior art, such a process would not seem very promising since the hydroxyl groups of the polyether or polyester generally have approximately the same reactivity with respect to NCO groups as water. Thus, it would be expected during the reaction of a polyisocyanate with water in a polyether or polyester for the NCO groups to react to about the same extent with the water and with the OH-groups of the polyol.
The separate production of urea-isocyanates, the isolation and purification thereof and the subsequent dispersion or dissolution of the urea-isocyanates in relatively high molecular weight polyols, such as are used for the synthesis of polyurethanes, is expensive. An object of the present invention is to synthesize urea isocyanates directly in the relatively high molecular weight polyols from aromatic isocyanates which are easily accessible in a simple and economic process, in order to be able to use these urea isocyanate/polyol dispersions or solutions directly for the synthesis of polyurethanes.